In the processing of pulp for bleachable grades, one of the first process steps following cooking is the removal of larger uncooked pieces of wood, commonly referred to as knots. The device used for this purpose is the deknotter, more commonly referred to simply as the “knotter”. This conventional nomenclature will be used here.
The knotter uses a barrier, or screen cylinder, with perforations in the 8 to 12 mm diameter range being most common, although perforations as small as 6 mm or as large as 16 have been used. The most common size is 9.5 mm diameter. Pulp stock passes through this screen cylinder, while the larger pieces of uncooked wood chips cannot pass through. Flows on the inlet side of the screen cylinder carry the knots to one end of the screen cylinder, from which they are discharged as “rejects”.
All modern knotters are vertically mounted, use cylindrical screen cylinders, and are inward flow (meaning the inlet pulp passes from the outside of the screen cylinder to the inside). One type of conventional knotter uses a rotating screen cylinder with a stationary set of hydrofoils on the accept side, while another uses a stationary screen cylinder with rotating hydrofoils on the accept side.
With both types of conventional knotters, feed pulp at about 5% outside diameter consistency is presented to the inlet connection tangential to the top chamber. There is a nozzle in this inlet connection to accelerate the flow, which then flows rotationally around the outside of the perforated screen cylinder.
Accepted pulp goes through the perforations in the screen cylinder toward the center of the machine. From there it passes either upward or downward and then outward through a tangential accept connection.
Pulp on the outside (inlet side) of the screen cylinder that has not yet passed to the accepts continues moving in a rotating flow around the outside of the screen cylinder and also traveling downward. As this flow moves downward, the energy in it dissipates and the movement slows. If this movement were to ever stop completely, the machine would plug.
To prevent this, in the conventional knotter design that uses a stationary screen cylinder with rotating hydrofoils on the accept side, at a point approximately ⅔ of the way down the screen cylinder, a jet of high velocity dilution (known as the “elutriation flow”) is injected tangentially along the wall of the chamber. The energy in this flow (which is equal to the flow times the velocity head) reinvigorates the stock flow and avoids any chance that the flow could stop and plug.
The elutriation flow is a much greater volume than the rejects flow, and most of this flow must pass into the accepts. It carries most of the pulp stock with it, leaving the reject consistency very low relative to the inlet consistency. This makes the job of the secondary knotter (the device which takes the knot-laden flow from the primary knotter and extracts the knots, discharging them finally in a damp state to a bunker or other disposal step) relatively easy.
In this design of machine, there are two specially designed hydrofoils on the inside of the screen cylinder there are two specially designed hydrofoils, mounted on a rotor and driven by a motor connected by V-belts, that produce an outward pulsation through the screen cylinder. It is the intent of these hydrofoils that they push any knots away from the inlet side of the screen cylinder so that the passing flow can carry them away.
Although the elutriation flow is fundamental to the operation of this design of conventional machine, this same flow also significantly dilutes the accept flow. Any dilution added to the pulp stream must be removed in the subsequent washing step in many instances, and the increased flow means that the equipment in that washing step must be larger, sometimes significantly so.
The solution used in the other type of conventional knotter is very similar, except to get around the need for the elutriation flow, the screen cylinder is mechanically driven and rotates. This has essentially the same effect as the elutriation does in the knotter that uses a stationary screen cylinder (minus the knot washing effect), but without the downside of diluting the pulp. These machines have stationary foils inside the rotating screen cylinder, which are fully analogous to the rotating foils of the conventional knotter with the stationary screen cylinder.